In a particle beam therapy, a diseased portion is irradiated with, for example, a proton beam or a carbon beam accelerated up to 70% of the light velocity. Such a high-energy particle beam has the following characteristics when irradiating into a tumor or the like in a body. Firstly, an irradiating particle beam stops almost at a penetration position proportional to the particle beam energy raised to the 1.7th power. Secondly, the energy density that is imparted to the path through which the irradiating particle beam penetrates until it stops in a body becomes maximum at the particle beam stop position. The energy density of the particle beam is referred to as a dose. A characteristic depth dose profile formed along the path through which a particle beam penetrates into a body is referred to as “Bragg curve”.
The position where the dose of the particle beam becomes a maximum value is referred to as “Bragg peak”. The particle beam scanning irradiation system scans a tumor so that the Bragg peak position is kept coincident with its three-dimensional shape. A peak dose at each scanning position is adjusted to form a three-dimensional dose distribution in a target (tumor portion) determined preliminarily by an imaging diagnosis.
A method of scanning irradiation positions with a particle beam includes a scanning method in the lateral directions (X- and Y-directions) substantially orthogonal to the irradiation direction of the particle beam and a scanning method in the depth direction (Z-direction) being the irradiation direction of the particle beam. In the lateral scanning, there are a method of moving a patient with respect to the particle beam and a method of shifting the position of the particle beam using an electromagnet or the like. The latter method using an electromagnet is generally employed.
Varying energy of the particle beam is only method for scanning in the depth direction. Two methods are conceivable for the energy variation: a method of varying the particle beam energy by an accelerator and a method of using an energy varying device called a range shifter installed in a beam delivery line or an irradiation line. Nowadays, the method using an energy varying device is widely employed. A range shifter may sometimes include a device called an energy selection system that performs energy analysis and momentum selection.
The method for lateral scanning of a particle beam is classified into two basic irradiation methods: a spot scanning irradiation method and a hybrid scanning irradiation method. In a spot scanning irradiation method, a particle beam is emitted and intensity of the particle beam is once weakened when an irradiation amount at a given irradiation position reaches a planned value (refer to Non-Patent Document 1). At this time, the particle beam intensity is generally set to zero. To irradiate a next irradiation position with the particle beam, a current value for the scanning electromagnet is changed and the particle beam intensity is increased again, and then the particle beam is emitted. Instead of increasing the particle beam intensity, re-extraction of the particle beam from the accelerator is also made.
In the hybrid scanning irradiation method, while its basic way of irradiating a planned position with the particle beam by a planned amount is the same as with the spot scanning irradiation method, the particle beam is scanned not with the irradiation being stopped but with the irradiation being continued when shifted to a next irradiation position (refer to Non-Patent Document 1).